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Replacing Cut-Outs with the Photon MLC for Standard Electron Treatments

S Mueller

S Mueller1*, M K Fix1 , D Henzen1 , D Frei1 , D Frauchiger1 , K Loessl1 , M F M Stampanoni2 , P Manser1 , (1) Division of Medical Radiation Physics and Department of Radiation Oncology, Inselspital, Bern University Hospital, and University of Bern, Switzerland, (2) Institute for Biomedical Engineering, ETH Zurich and PSI, Villigen, Switzerland


TH-CD-708-10 (Thursday, August 3, 2017) 10:00 AM - 12:00 PM Room: 708

Purpose: To investigate the accuracy of a corresponding Monte Carlo (MC) beam model for photon multileaf collimator (pMLC) shaped electron beams and to compare the dosimetry of pMLC and cut-out collimated electron beams for standard electron treatments.

Methods: Calculated and measured depth dose curves and dose profiles in water of pMLC collimated electron beams with SSDs of 70, 80, 90 and 100 cm and a field size at water surface of 4.2 x 4.2 cm2 and 9.8 x 9.8 cm2 for all available energies of a Varian Novalis Tx equipped with a HDMLC are compared in units of cGy/MU for validating a multiple source MC beam model connected to a macro MC algorithm. Dose distributions of plans for cases of a left and a right breast boost, testis and sternum with a single electron field collimated either by pMLC or by cut-outs are compared.

Results: All calculated and measured depth dose curves and dose profiles used for validating the MC beam model agree within 3% of the maximal dose value or 2 mm distance to agreement. Dose distributions of the pMLC and cut-out plans are similar in terms of dose homogeneity and OAR sparing for the left and right breast boosts and the testis case, while for the sternum case the broader penumbra of the pMLC shaped electron beam leads to a moderate increase of the mean dose to the lungs compared to the cut-out plan. However, clinical constraints are still fulfilled for this case.

Conclusion: The pMLC is an alternative for collimating electrons beams for standard electron treatments with appropriate dose distributions and without the disadvantages in clinical workflow and delivery connected with conventional cut-outs. Dose distributions of pMLC shaped electron beams are accurately calculated using an MC beam model. This work was partly supported by Varian Medical Systems.

Funding Support, Disclosures, and Conflict of Interest: This work was partly supported by Varian Medical Systems.

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